It is well known that acuity is impaired by factors such as reduced stimulus contrast, viewing eccentricity, and interference from nearby contours (crowding). Despite numerous studies, a comprehensive quantitative characterization of these effects, including any interactions, remains lacking.

We measured threshold letter size (acuity) for 150-ms tumbling-E targets at the fovea and lower visual field at eccentricities of 3, 5, and 10° for 5 subjects. Targets were presented in isolation or flanked on all four sides by randomly-oriented tumbling-Es at six center-to-center spacings (1.2-5x the letter size). Weber contrast of the entire stimulus varied from -2.5% to -99% in eight logarithmic steps.

We find that across subjects, contrasts, and eccentricities, flanked acuity can be well described as a function of nominal flanker distance. A parsimonious characterization is a two-line fit in log-log space comprising a flat portion where flankers do not affect target recognition, and, within the absolute critical spacing for crowding, a line with constrained slope of -1. Except where crowding is essentially absent (foveally), average r[sup]2[/sup] for this fit is 0.85±0.17. Interestingly, contrast reduction causes only a modest (<1.5x) increase in the critical spacing, instead primarily elevating the flat portion of these curves.

For all combinations of contrasts and flanker spacings, threshold acuities are linear functions of eccentricity (average r[sup]2[/sup]=0.94±0.069). The critical spacing is also linearly related to eccentricity (average r[sup]2[/sup]=0.98±0.038). Finally, a two-line fit relating acuity to contrast is consistent across subjects and eccentricities (average r[sup]2[/sup]=0.88±0.048), yielding a power-law with exponent of -0.55 for contrasts below approximately 20%.